1. FIELD OF THE INVENTION
The invention relates to a method in a tandem cold mill for rolling on gauge material in a workpiece, such as strip or sheet during threading and tailing out by varying the interstand tension in the workpiece. More specifically, the invention relates to employing an existing delivery automatic gauge control (AGC) in a tension mode, to change the speed of the downstream stands relative to the last to vary the interstand tension to obtain an on gauge length for the workpiece travelling through the mill when the mill is being operated at low speed.
2. DESCRIPTION OF THE PRIOR ART
Presently, in the threading and tailing out processes of a multi-stand tandem cold rolling mill for reducing a workpiece, such as strip, most of the out of tolerance or off gauge strip occurs during these low mill speeds, with very little or no off gauge strip occurring in the body of the strip at the full run of the mill when the mill is operating at a high speed.
During the low mill speeds, the roll gap of each stand is set at a desired setting and the reduction in the strip is being determined through the "tension by speed" mode of the mill where the interstand tension regulators control the speeds of the stands. The tension by speed controllers make stand speed corrections which are added to the mill operator's stand speed settings. When the stand speed settings are changed, there will be off-gauge strip leaving the mill since the mill speed setup has been modified by the strip tension by speed regulators. During low mill speeds when all the mill interstand tension regulators modify stand speed to regulate the strip tension to a strip tension setting determined by the mill operator, the only way possible to control strip delivery gauge by a delivery automatic gauge control system which monitors the strip thickness leaving the mill, is for this delivery AGC system to change the strip interstand tension in a manner to bring the delivery strip gauge (thickness) within tolerance. There is a limit to which the delivery AGC system by tension is permitted to change the interstand strip tension relative to the operator strip tension setting of the mill operator which is typically .+-.40% of the operator's setting. Usually, the interstand AGC by tension system is saturated during the low speed of the mill in making all the strip tension connections permitted. Even with this situation, the strip delivery gauge can still not be within tolerance.
When the last stand of a mill has sandblasted rough rolls, which is the case for a sheet mill, the interstand strip tension between the last two stands is usually regulated by changing the speed of one or both of these stands regardless of the mill speed. When this is the case, the interstand strip tension between the last two stands is changed by a delivery AGC system to bring the delivery strip on gauge. Again the amount of tension change permitted in the strip is typically .+-.40% of the operator's strip tension setting. If the delivery AGC by tension system is energized at all times during the rolling process, that is, at both low and high mill speeds, the delivery AGC by tension system usually goes into a saturation state. When the delivery AGC by tension controller is in this saturation state making the maximum strip tension corrections permitted, it is impossible for the delivery AGC to make any further delivery gauge corrections. This quite often happens, i.e. the controller becomes saturated at high mill speeds, and remains saturated when the mill speed is changed to a lower speed. At high mill speeds, there is another delivery AGC system which is available which can control the delivery gauge in the strip. This is known as a delivery AGC by speed system.
The delivery AGC system by tension is not required when this delivery AGC by speed system is in operation. Present mill practice is to keep both the delivery AGC by speed and the delivery AGC by tension systems in operation at high mill speeds. Presently in existing mills the delivery AGC by tension system is not energized selectively where it is only turned on at low mill speeds and also not all the interstand strip tensions are changed to make delivery gauge corrections at low mill speeds, such as threading and tailing out. Unfortunately, this practice and operation of the mill does not prove to be adequate to produce on gauge strip relative to the body of the strip being reduced in the full run of the mill.
Some examples illustrating the relationship between automatic gauge control and speed and tension regulators for gauge control are shown in U.S. Pat. Nos. 3,740,983; 3,765,203; 3,768,286; 3,848,443; 3,782,151; 4,011,743; 4,016,735; and 4,286,447.
In the full run of the mill, strip within gauge tolerance is obtained through the operation of the "tension by roll gap" mode where the interstand tension regulators control the roll gap of the stands, and through the selective operations of the entry and the delivery automatic gauge control (AGC) systems. The delivery AGC system generally uses an X-ray gauge at the delivery side of the last stand for monitoring deviations in the strip and may consist of what is referred to as an "AGC by speed" mode and an "AGC by tension mode." The "AGC by speed" mode generates the speed changes in the downstream stands, and the "AGC by tension" mode generates the tension changes between stands.
In the operation of the mill, when the mill is accelerated from the low mill threading speed to the normal high mill full run speed, the strip usually goes thin. When the mill is decelerating from the high run speed to the low tailing out speed, the strip has a tendency to go thick. This phenomena is known in the industry as speed effect. When the mill is accelerating, the automatic gauge control (AGC) systems may be fully functional and the interstand tension regulators are generally changed from the tension by speed mode to the tension by gap mode. The AGC systems make stand speed corrections to correct the delivery gauge of the strip which usually is thick. The AGC corrections are made in a direction to correct for thick strip which means that the AGC corrections make the strip go thinner. In the meantime, the speed effect occurs causing the strip to go even thinner. The net result is that the strip can become too thin. In the meantime, the strip is accelerating to the high run speed before the delivery AGC system can bring the strip back on gauge. Since the strip is going through the mill at a high rate of speed before the strip gets on gauge, a great amount of strip will not be within gauge tolerance.
When the mill is decelerating, that is, when the mill speed is going from high to low, the automatic gauge control systems generally are still operating and the interstand tension is changed from the "tension by gap" mode to the "tension by speed" mode for low mill speed. Due to the speed effects and to the instability and/or saturation of several of the systems for gauge control of the strip in the tailing out phase of the mill, the strip tends to become too thick.
Ideally, if the strip is brought on gauge as soon as possible on the head end during threading, and on the tail end during tailing out, there will be less off gauge strip.